Prior art describes liquid crystal displays (LCD), and liquid crystalline light modulation media, which are in the optically isotropic blue phase when being operated in a display application. Such displays are disclosed for example in DE 103 13 979 A1 or the corresponding applications WO 04/046805 A1 and US 2006/0050354 A1. The operating voltage of these media is almost temperature-independent in the blue phase region, while it dramatically increases with increasing temperature in the isotropic phase (Kerr effect). However, the limitation for the application of this type of displays is the insufficient temperature range of the blue phase.
Blue phases are typically observed in highly chiral single mesogenic compounds or mixtures but generally over a very small range—typically less than 1° C. In order to operate novel fast switching display modes that operates in the blue phase, it is necessary to have a mixture that possesses as wide a blue phase as possible, typically from −20 to +60° C.
Other typical liquid crystal properties necessary for display applications are also required.
To achieve broader blue phases, prior art suggests to add polymerisable compounds to the liquid liquid crystalline medium which are then polymerised in-situ. The polymer or polymer network thereby formed is reported to stabilise the blue phase of the liquid liquid crystalline medium.
Kikuchi, H. et al., Polymeric Materials Science and Engineering (2002), 1(1), 64-68 and Kikuchi, H. et al., Polymeric Materials Science and Engineering, (2003), 89, 90-91 describe in-situ polymerisation to stabilise the blue phase over a temperature range that is 60° C. wide and includes ambient temperature (−67 to 53° C.) and, respectively, over a temperature range that is 120° C. wide and includes room temperature (−73 to 53° C.). This is achieved by using a non-mesogenic mono-reactive monomer, such as 2-ethyl hexyl acrylate, together with a difunctional reactive mesogen (RM), e.g. RM257 which is available from Merck KGaA, in a nematic liquid liquid crystalline host mixture.
H. Kikuchi et al., Nature Materials (2002), 1(1), 64-68 describes in-situ polymerisation to stabilise the blue phase over 60° C. including room temperature (−67 to 53° C.) using RM257 and a non-mesogenic reactive monomer such as 2-ethyl hexyl acrylate.
JP 2003-327966 A describes a composite liquid crystal for use as optical modulation element with specific composition, comprising a low molecular weight LC, which possesses a blue phase, and a polymer network within this medium obtained by polymerisation of non-liquid crystalline monomers alongside with a cross-linking agent RM257. In particular, the preferred embodiment is for branched monoacrylate monomers with alkyl groups in the side-chain.
WO 2005/090520 A1 describes an ‘invisible’ polymer stabilised blue phase and again specifically mentions that non-liquid crystalline monomers are preferred.
WO 2005/080529 A1 describes polymer stabilised blue phase using mono- and multireactive monomers, but uses only non-mesogenic monoreactive monomers.
The above methods and systems according to prior art do specifically mention that they utilise either a non-mesogenic monomer or a combination of RM257 and a non-mesogenic monomer. However, these systems have significant drawbacks. In particular by using non-mesogenic monomers, the structure of the forming polymer has significantly different physical properties from the monomers and the liquid liquid crystalline host mixture. There is a need to stop and change the temperature at which the photopolymerisation is done many times, thus making this method a difficult and time-expensive process to undergo and difficult to reproduce.
In addition, the use of non-mesogenic monomers such as 2-ethylhexylacrylate can be problematic due to the volatile nature of the compound, leading to problems of loss due to evaporation and inhomogeneity of the mixed monomer/host system.
Also, the use of non-mesogenic compounds can severely lower the clearing point of the liquid liquid crystalline host, leading to a much smaller width of polymer stabilised blue phase, which is not desirable for display applications.
Moreover, the above prior art documents describe liquid liquid crystalline media that are based on a crude cyano-based mesogenic host, which has a poor high voltage holding ratio (VHR).
There is therefore a need to find materials that stabilise the blue phase over a wide temperature range, and which possess desirable properties such as fast switching, good voltage holding ratio, low voltage, high clearing point, and high stability to light and temperature.
The invention has the aim of providing improved methods and materials, in particular new RMs and liquid liquid crystalline mixtures comprising them, to achieve polymer stabilised blue phases, which do not have the above-mentioned disadvantages of methods and materials described in prior art. Another aim of the invention is to extend the pool of RM materials available to the expert. Other aims are immediately evident to the expert from the following description.
Surprisingly, it has been found that by using an RM comprising a cyclohexylene core, a stabilised liquid liquid crystalline blue phase which has a broad temperature range, extremely fast switching times, very high voltage holding ratio and low voltage can be achieved.
In addition, by using a total RM system (i.e. all the monomers are rod-like) a high clearing point can be maintained, and polymerisation can be carried out at a single temperature, enabling a production friendly process.
The cyclohexylene core of the RM also appears to be stable to the UV light used in the polymerisation process. The resultant polymer stabilised blue phase therefore has a high voltage holding ratio (VHR).
Also, it has been found that by using cyclohexylene RMs in combination with a liquid liquid crystalline host comprising fluorophenyl liquid liquid crystalline compounds, the RMs do effectively stabilise this host to give a high VHR, which is necessary for state-of-the-art LCDs.
JP 2005-15473 discloses RMs of a broad formula and does also disclose some RMs comprising a cyclohexylene ring, however, compounds as claimed in the present invention are not shown.